Ion Cyclotron and Heavy Ion Effects on Reconnection in a Global Magnetotail R
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, A09206, doi:10.1029/2004JA010385, 2004 Ion cyclotron and heavy ion effects on reconnection in a global magnetotail R. M. Winglee Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA Received 12 January 2004; revised 21 May 2004; accepted 8 July 2004; published 22 September 2004. [1] Finite ion cyclotron effects play a significant role in determining the dynamics of the neutral sheet. The demagnetization of the ions facilitates reconnection and produces an electric field perpendicular to the direction of the tail currents. This in-plane electric field drives field-aligned currents and an out-of-plane (or core) magnetic field in conjunction with the generation of flux ropes. In addition to these electromagnetic effects, it is shown that ion cyclotron effects lead to the preferential convection of plasma from the dawnside to the duskside. This convection is consistent with results from single- particle tracking but differs from ideal MHD treatment where the flow occurs symmetrically around the Earth. A physical manifestation of these asymmetric particle trajectories is the wrapping of the field-aligned current between the region 1 currents and the region 2 and/or region 0 currents. In addition, localized density enhancements and depletions are seen in the tail where the local heavy ion density can be substantially elevated over ionospheric conditions. Because of the local density variations, reconnection across the tail is inhomogeneous. Reconnection is initiated postmidnight and then sweeps across to the dawn and dusk flanks within a few minutes. Because of this spatial variation, the ejection plasmoid is actually U-shaped and the subsequent flux rope formation is highly skewed. INDEX TERMS: 2744 Magnetospheric Physics: Magnetotail; 2753 Magnetospheric Physics: Numerical modeling; 2788 Magnetospheric Physics: Storms and substorms; 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; 2431 Ionosphere: Ionosphere/ magnetosphere interactions (2736); KEYWORDS: reconnection, multifluid, simulation, magnetosphere, magnetotail, flux ropes Citation: Winglee, R. M. (2004), Ion cyclotron and heavy ion effects on reconnection in a global magnetotail, J. Geophys. Res., 109, A09206, doi:10.1029/2004JA010385. 1. Introduction and Hughes, 1992]. Lepping et al. [1995, 1996] used a force-free model for flux ropes and showed that in both the [2] Magnetic reconnection plays a critical role in deter- middle and distant tail regions the average diameter was mining the morphology of the magnetosphere during sub- 10 Re and in several instances may be as small as 1–3 Re. storms and storms. Evidence of reconnection has been The origin of these flux ropes in the magnetotail have been growing since the initial observations of negative Bz in attributed to the currents that are generated when the ions the tail associated with substorm onset [e.g., Hones, 1976]. become unmagnetized, while the electrons remain magne- While initial models showed a well-defined O-type neutral tized in the reconnection region. The demagnetization of the line within the plasmoid [Hones, 1979], subsequent studies ions leads to the violation of the frozen-in theorem con- have shown that plasmoids can have an appreciable core ditions and provides the physical mechanism by which magnetic field [Sibeck et al., 1984; Elphic et al., 1986; reconnection can occur. This resultant magnetic diffusion Slavin et al., 1989; Sibeck, 1990; Moldwin and Hughes, region is thought to have been observed during a Wind 1991, 1992]. On occasion, the core magnetic field can be as perigee pass [Øieroset et al., 2001]. strong as, and in some cases exceed, the lobe magnetic field [4] Theoretical and modeling efforts have increasingly [Slavin et al., 1995]. The direction of the core magnetic focused on resolving the microstructure of the reconnection field is usually (34 out of 39 cases) in the direction of the By region and thereby quantifying the controlling influences of component of the interplanetary magnetic field (IMF) the system. Drake et al. [1994] have shown for idealized [Moldwin and Hughes, 1992]. geometries that reconnection does not occur in the smooth [3] The core field is also observed to be generally MHD manner but rather occurs through filamentation and unidirectional, although, on occasion, bipolar By signatures kinking of the current sheet. Winglee and Steinolfson are seen in conjunction with bipolar Bz signatures [Moldwin [1993], Steinolfson and Winglee [1993], Biskamp et al. [1995], Ma and Bhattacharjee [1996], Zhu and Winglee Copyright 2004 by the American Geophysical Union. [1996], Pritchett et al. [1996], Shay et al. [1998], and Birn 0148-0227/04/2004JA010385 et al. [2001] have demonstrated, using a variety of different A09206 1of17 A09206 WINGLEE: ION CYCLOTRON AND HEAVY ION EFFECTS A09206 codes (including full particle, hybrid, and multifluid), that symmetrically around the Earth. If we are to better model the Hall term in the generalized Ohm’s law plays an the dynamics of the magnetosphere, then this discrepancy important role in controlling the reconnection rate in colli- between particle and fluid perspectives needs to be resolved. sionless plasmas. The importance of the Hall term is that it [9] In section 2 the formalism behind a fully self-consistent describes the electromagnetic response to the system as the multifluid treatment with ion cyclotron terms included in the ions become demagnetized owing to their large ion gyrora- electrodynamics as well as the plasma dynamics is described. dius relative to that of the electrons, which remain magne- Note that it is these latter ion cyclotron terms that produce tized. As this demagnetization occurs, the above models all differential convections between the different ion species. show a highly structured reconnection region with a strong While the global code does not have the capacity to resolve all out-of-plane (or core) magnetic field. While the different the structures seen in high-resolution hybrid codes, it is models can show a variety of different instabilities (includ- demonstrated that Hall and ion cyclotron effects that are ing firehose, ballooning, lower hybrid drift, and kink- incorporated in the global simulations are already sufficient tearing), they share the common feature of a highly to alter the dynamics of the global magnetosphere. + structured magnetic field mapping that goes well beyond [10] The presence of heavy ions such as O also adds a that seen in ideal MHD models. new inherent scale length to the reconnection region. This [5] While the above particle and hybrid models are very scale length is fully resolved within the global code. In informative in determining the local structure of the recon- section 3 we demonstrated that while the overall magnitude nection, the one real problem that has received very little of the cross-polar cap potential and field-aligned currents attention is how to place this physics into global models so remain of the order of that seen in previous simulations, the that direct comparisons with in situ observations of magne- full inclusion of the ion cyclotron terms modifies the tospheric activity can be made. Initial global simulations structure of the auroral current system to produce the spiral [Winglee et al., 1998] incorporating the Hall and rPinthe flow of current between the region 1 and region 2 currents Ohm’s law have shown similar core magnetic field and flux as noted by Iijima and Potemra [1976, 1978] using data ropes structures, as seen in the above particle and hybrid from the Triad satellite. In other words these apparently codes but on the observed scale size of a few Re. This initial small corrections have global consequences for magneto- work assumed the presence of only a single ions species spheric activity. In section 4 we then show the actual within the magnetosphere. structures generated in the tail including the dawn-dusk [6] However, the assumption of a single ion species for acceleration of plasma and asymmetric reconnection rate the magnetosphere is probably not accurate, since during across the tail, with a corresponding distortion of the disturbed times the ionosphere can be an important source plasmoid/flux rope topology. A summary of results is given of plasma to the magnetotail [Chappell et al., 1987] and this in section 5. outflow can be dominated by heavy ionospheric ions such + as O [e.g., Yau and Andre´, 1997]. To address this issue, 2. Simulation Model Winglee [2000] developed a global model that included the presence of heavy ions. This model as able to provide three- [11] In this section we detail the equations used in the dimensional (3-D) rendering of the geopause (i.e., the multifluid modeling, including for the first time ion cyclo- boundary within the magnetosphere where the plasma tron corrections to the momentum equations of the different changes from one being dominated by plasma of solar wind ions species present. A discussion is presented on the origin to one being dominated by plasma of ionospheric relative size of these corrections and why they can at least origin). More recently, these ionospheric outflows have partially be incorporated into a global model. We also been shown to produce saturation of the cross-polar cap compare and contrast the code capabilities relative to hybrid potential [Winglee et al., 2002]. codes to give more insight into the assumptions being made [7] In this paper we demonstrate that ion cyclotron effects within the code. The code details are then described. are not only important with respect to the electrodynamics 2.1. Electrodynamics of the system due to corrections in the generalized Ohm’s law, but they also have an important effect on the momen- [12] The dynamics of any fluid plasma component are tum equations for the different ions species. These effects given by are shown to play a critical role in determining the particle acceleration and convection in the reconnection region and @ra þrÁðÞ¼raV a 0 ð1Þ in the structure of the field-aligned currents into the auroral @t region.